Heretofore, magnetic recording materials have been primarily used as recording materials embedded in cards such as credit or bank cards. Such magnetic recording materials have the merit of being able to read and write information easily, but problems associated with them are that the information may be relatively easily falsified and high-density information recording is unachievable.
Recently, IC or optical cards are being under development with the recording and reproduction of high-density information in mind. In particular, optical cards relying upon optical recording/reproduction systems have an advantage of recording information at much higher densities over those to which conventional magnetic or IC systems are applied.
In this connection, the optical recording media or materials so far proposed for optical cards involve the following.
The recording medium shown in FIG. 19(A) as an example is a so-called ROM (read only memory) type of optical recording medium. In this instance, an optical card 50 is provided thereon with an optical recording medium comprising a portion 50 of high reflectivity and a portion 52 of low reflectivity, said portion 52 of low reflectivity defining an information recording bit.
FIG. 19(B) is a sectional view of the optical card 50. As illustrated, a transparent substrate 54 is provided thereon with a light reflecting material 55 corresponding to an information recording pattern by photolithography, while a substrate material 58 having a black printed layer 57 thereon is joined thereto through an adhesive layer 56, thereby forming the optical card 50. In this instance, therefore, the information is read out by detecting a difference in the light reflectivity between the light-reflecting material 55 and the printed layer 57.
FIG. 20 shows an example of the ROM type of optical recording medium disclosed in Japanese Patent Publication No. 58-48357. In this instance, a film 60 of a silver halogenide emulsion formed on a certain substrate material is subjected to pattern exposure corresponding to the information to be recorded and the developed, thereby forming an information recording bit 62 differing in light reflectivity from a surrounding portion 61.
FIG. 21 illustrates an example of the so-called DRAW (direct read after write) type of optical recording medium wherein no development treatment is needed after information has been written and the information can thus be read directly after writing. In this instance, a photopolymer 71 is formed on its one side into an irregular or modulated plane, along which an optical recording material 72 such as Te is provided in the form of a thin film. That film is bonded to a substrate 74 with a bonding agent 73, and a light transmitting acrylic resin 75 or the like is then laminated on the upper side of the photopolymer 71, thereby forming an optical recording medium.
Writing of information on such an optical recording medium may be achieved by scanning the modulated plane of the recording medium with a laser beam, 76 irradiated as illustrated in FIG. 22 to ensure that e.g., the convex portions are in alignment therewith and, thereafter, writing bit information on the convex portions.
FIG. 23 illustrates a DRAW type of optical recording medium disclosed in Japanese Patent Publication No. 58-48357. In this instance, a film 80 of a silver halogenide emulsion is formed on a substrate and then subjected to pattern exposure and development, thereby forming portions 81 and 82 different in light reflectivity from each other. After the position of the portion 81 has been ascertained by a difference in light reflectivity, bit information is written on the portion 81.
It has also been proposed to use such an optical recording medium as a card and to apply a separately prepared hologram onto the surface thereof.
However, such conventional optical recording media are all so relatively sophisticated to make that they are not always economical for mass-production or duplication. Nor are the obtained optical recording media satisfactory in terms of their shelf stability and durability over time.
Referring to the optical recording medium shown in FIG. 19 as an example, patterning of the light reflecting material is carried out by wet etching with a strong acid or the like. Washing and drying of the etching solution are then troublesome and time consuming. If washing is insufficient, then the completed optical recording medium is likely to suffer from accelerated corrosion with the passage of time, resulting in a reduction of its life. Usually, the etching solution does not only involve difficulty in its pH control but also poses a problem due to foreign matters contained therein.
A problem with the optical recording medium shown in FIG. 20 is that it is difficult to control the convex plane to be formed, because a predetermined minimum level of accuracy is necessarily imposed upon its height and width. This leads to another problem that the manufacturing process is so complicated that the cost increases.
The optical recording medium shown in FIG. 23 may possibly be easier to make than that shown in FIG. 20 in view of the fact that only the accuracy of the track width is critical. However, a cost problem again arises due to the need for a specific photographic material such as a special silver halogenide emulsion.
If the optical recording media prepared by any one of such processes is applied to an optical card, a problem then arises that the information recording pattern may be counterfeited by optical procedures, because it can be easily detected under a microscope or the like. However, although an optical card applied on its surface with a separately prepared hologram has an advantage of its hologram portion being indecipherable under an electron microscope, yet it is likewise less useful for fake-proof purposes, because the hologram can be duplicated by templating or optical procedures remaining exposed to the surface and easily delaminated.